81. _Sun Time and Clock Time._--It is noon by the sun when the sun is on the meridian, and by the clock at the middle of the civil day. Now, as the civil days are all of the same length, while solar days are of variable length, it seldom happens that the middles of these two days coincide, or that sun time and clock time agree. The difference between sun time and clock time, or what is often called _apparent solar time_ and _mean solar time_, is called the _equation of time_. The sun is said to be _slow_ when it crosses the meridian after noon by the clock, and _fast_ when it crosses the meridian before noon by the clock. Sun time and clock time coincide four times a year; during two intermediate seasons the clock time is ahead, and during two it is behind. * * * * * The following are the dates of coincidence and of maximum deviation, which vary but slightly from year to year:-- February 10 True sun fifteen minutes slow. April 15 True sun correct. May 14 True sun four minutes fast. June 14 True sun correct. July 25 True sun six minutes slow. August 31 True sun correct. November 2 True sun sixteen minutes fast. December 24 True sun correct. One of the effects of the equation of time which is frequently misunderstood is, that the interval from sunrise until noon, as given in the almanacs, is not the same as that between noon and sunset. The forenoon could not be longer or shorter than the afternoon, if by "noon" we meant the passage of the sun across the meridian; but the noon of our clocks being sometimes fifteen minutes before or after noon by the sun, the former may be half an hour nearer to sunrise than to sunset, or _vice versa_. The Year. 82. _The Year._--The _year_ is the time it takes the earth to revolve around the sun, or, what amounts to the same thing, _the time it takes the sun to pass around the ecliptic_. (1) The time it takes the sun to pass from a star around to the same star again is called a _sidereal year_. This is, of course, the exact time it takes the earth to make a complete revolution around the sun. [Illustration: Fig. 96.] (2) The time it takes the sun to pass around from the vernal equinox, or the _first point of Aries_, to the vernal equinox again, is called the _tropical_ year. This is a little shorter than the sidereal year, owing to the precession of the equinoxes. This will be evident from Fig. 96. The circle represents the ecliptic, _S_ the sun, and _E_ the vernal equinox. The sun moves around the ecliptic _eastward_, as indicated by the long arrow, while the equinox moves slowly _westward_, as indicated by the short arrow. The sun will therefore meet the equinox before it has quite completed the circuit of the heavens. The exact lengths of these respective years are:--